1. Field of the Invention
This invention relates to an image forming process employing an electrophotographic photosensitive member, and more particularly to an image forming process to effectively maintain the light area potential of the photosensitive member at a substantially constant value during repeated operations thereof.
2. Description of the Prior Art
Selenium, cadmium sulfide, zinc oxide, etc. have so far been known as inorganic photoconductive materials for use in electrophotographic photosensitive members. On the other hand, organic photoconductive materials have been proposed including a variety of photoconductive polymers, the first of the polymers being polyvinylcarbazole, and a number of low-molecular photoconductive substances such as hydrazones, pyrazoline compounds, oxadiazole compounds, indole compounds, carbazole compounds, anthracene, and pyrene.
Organic photoconductive materials are generally superior to inorganic photoconductive materials in light weight, coating-ability, etc. However, since their sensitivity is insufficient, various methods have been offered for sensitizing them. An effective one of such methods is to improve the sensitivity by allotting the functions of a photosensitive member to two contiguous layers, one generating charge carriers on light irradiation and the other transporting these charge carriers, as disclosed, for example, in U.S. Pat. Nos. 3,837,851 and 3,871,882.
It is necessary in this separate-function type of photosensitive layer that the charge carriers injected into the charge transport layer from the charge generation layer are transported to the neighborhood of the surface of the charge transport layer in an electric field without being trapped in the course of the transport layer. In practice, however, a number of repetitions of charging and exposing this type of photosensitive member raise the light area potential, which is conceivably caused by the accumulation of the trapped carriers.
According to studies of the present inventors, a charge transporting material contained in the charge transport layer undergoes an irreversible reaction, particularly in the vicinity of the surface, by absorbing ultraviolet or visible light; presumably, the irreversible reaction product may trap and accumulate charge carriers injected from the charge generation layer; thus the light portion potential gradually increases on repeated use of the photosensitive member.
Such phenomenon can be observed when the photosensitive layer is exposed to an environment including, for example, ultraviolet rays. In practice, when the photosensitive member is placed under exposure to external light including ultraviolet rays even for an instant, before being assembled in a copying machine and when thereafter it is mounted in the copying machine to be subjected to the usual image forming process including repetitive charging and exposing, the light area potential tends to gradually to result in images with considerable fogging.